There are known some devices for forming thin films of materials of various sorts on the basis of techniques such as those of CVD (Chemical Vapor Deposition), plasma CVD, optical CVD, and epitaxial growth, etc. In these devices a technique to mount and heat various wafers by making use of a heating system in which radio frequency (hereinafter referred to as RF) induction or optical excitation is employed. Such a technique is adapted to heat a supporter of the wafer by RF induction or optical excitation to thereby further heat the wafer supported by the supporter through heat conduction and thermal radiation. Since much gas is released from a heating supporter a prior susceptor has been structured in such a way that the heating supporter is coated with a dense coating film, through which any gas released by the heating supporter is unlikely to pass through the entire surface of the prior art supporter. Accordingly, any impurity gas which invades the heating supporter upon manufacture of that supporter is usually insufficiently removed therefrom even with degassing effected prior to film formation onto a wafer owning to the presence of the aforementioned dense coating film, and thereby causes the supporter to pollute a reaction chamber. Such a structure thus makes a clean process impracticable as the result of the above-described difficulty. Such a clean process is essential in putting future submicron patterns into practical use. This has been already described in detail in the specification of the application by the present inventor (Japanese Patent Application No. 60-211643. An Apparatus for Treating A Wafer). In what follows, the importance of such a clean process will be described, taking the case of epitaxial growth of silicon.
The progress and development of image sensing and LSI technologies are very rapid, and high sensitivity image sensors have been fabricated in the image sensing field together with DRAMs of 1M bits or more in the LSI field. In order to fabricate high performance electronic devices of this type, a high performance fabrication process is required as a matter of course, which process is thereupon not effected by indefinite factors and has excellent controllability.
A highly clean process for silicon epitaxial growth is an example of such a process. The highly clean process of this type is mainly needed to assure semiconductor devices in which the life times of minority carriers are long. In particular, such a highly clean process is effective for reducing various lattice defects involved on an epitaxial film.
The essential requirement to realize the highly clean epitaxial growth process for fabrication of high performance semiconductor devices is to remove almost all unnecessary gas components other than those required for attendant reaction from the atmosphere in a reaction chamber in which the process proceeds. That is, an ultracleaning process is essential to the realization of the highly clean epitaxial growth process indispensable to the manufacture of high sensitivity image sensors and ultrafine LSIs.
The epitaxial growth of silicon is, as expressed by for example: EQU SiH.sub.4 .fwdarw.Si+2H.sub.2 ( 1) EQU SiH.sub.2 Cl.sub.2 +H.sub.2 .fwdarw.Si+2HCl+H.sub.2 ( 2)
achieved by thermal decomposition of silane (SiH.sub.4) (formula (1)) and a hydrogen reducing reaction of dichlorosilane (SiH.sub.2 Cl.sub.2) (formula (2)).
However if the reaction atmosphere includes water (H.sub.2 O), nitrogen (N.sub.2), oxygen (O.sub.2), carbon monoxide (CO), carbon dioxide (CO.sub.2), chlorine (Cl), hydrogen chloride (HCl), hydrocarbon (C.sub.3 H.sub.3, etc.), and heavy metals (Au, etc.), then nitrogen, oxygen, carbon, chlorine, heavy metals (Au, etc.) and the like will invade into an epitaxial film to form an electrically deep level. In order to prevent this deep level from being formed, it is necessary to highly clean the reaction atmosphere.
The reaction atmosphere thus made extremely clean can prevent unnecessary impurities from invading into the epitaxial film. Namely, there is produced little density of the deep levels in a semiconductor with reduced unnecessary impurities to assure an epitaxial film with minority carriers each having a long life time in a semiconductor device.
Such a clean reaction atmosphere enables, as described above, an epitaxial film to be grown with minority carriers which have a long life time.
In order to make the reaction atmosphere clean, a gas supply system leading from a raw gas cylinder (or liquefied gas vessel) to a reaction chamber, the reaction chamber itself, and a gas exhaust system, must be cleaned together.
The major conditions to satisfy the above-described requirement are as follows:
1. Raw gases should be highly pure as much as possible.
2. For the gas supply system, reaction chamber, and exhaust system,
(a) any leakage of gas from the outside should be reduced to the utmost to result in no pollution from the atmosphere; and
(b) released gas from the piping system and pipe walls of the reaction chamber should be sufficiently reduced.
In other words, the materials for constructing the piping system and the reaction chamber should not contain any gas. In addition, the surface of any pipe wall should be sufficiently flat, and not have the properties of any layer changed upon processing for the purpose of a sufficient reduction of adsorbed gas. At the same time, an apparatus should be so provided as to adequately prevent the interiors of the reaction chamber and the gas supply piping system from being exposed to the atmosphere.
(c) any dead zone of the system in which any gas resides should be eliminated, and
(d) production of particles should be reduced as much as possible, and
if any movable member is required, any slideable part in the interiors of the gas supply system and the reaction system should be avoided, wherever practicable.
Some of these conditions may possibly be achieved by proper control or manipulation, but production of impurity gas from elements which constitute the reaction chamber is a problem of the arrangement of each of those elements in the reaction chamber, and hence the construction of those elements thereof is critical.
In this situation, it is particularly important to provide a susceptor which supports and heats a wafer in the reaction chamber upon the growth of an epitaxial film. For this, a structure is needed which is capable of sufficiently degassing the susceptor before the growth.
Conventionally, in order to reduce released gas from the susceptor upon the growth of an epitaxial film, a dense coating film (e.g., SiC film) is deposited over the entire surface of a heating supporter (e.g., a sintered body of carbon) of the susceptor involving large amounts of impurities. It is however difficult, with only this deposition method, to completely prevent any impurity gas from being freed from the heating supporter with the aid of the dense coating film under high temperature conditions during the growth of an epitaxial film. Therefore, the susceptor is usually subjected to a degassing treatment by heating it in a state of high vacuum or in a high purity gas before film formation. However, it is difficult for such an arrangement, in which the dense coating film is deposited over the entire surface of the heating supporter of the susceptor, to remove impurities contained in the heating supporter by the aforementioned degassing treatment before film formation.
In view of the drawbacks of the prior techniques, it is an object of the present invention to provide a wafer susceptor apparatus capable of removing impurity gas contained in a heat release supporter with the aid of a gas discharge part.